1. Field of the Invention
The invention is in the field of electric heating of a closed, controlled environment by heating elements of equal value which are requird to consume approximately equal power when selectively energized by independent power sources of differing voltages without altering the electrical connection of the heating elements.
2. Description of the Prior Art
Prior efforts to make an electric heating system compatible with alternative power sources of differing voltage have included the use of transformers, phase control devices, and dual networks of resistors. The use of a transformer substantially increases the weight, size, and cost of the system. Phase control methods increased the size and cost of the system, resulted in less efficient use of the power, and created radio frequency interference with nearby radio and electronic equipment. A dual network of resistors doubled the size and cost of the heating elements. The present invention avoids such increased size, cost, weight, inefficiency, and interference; it also eliminates the need for switches or other active components to change the circuit configuration of the resistors, other than means to connect the different power sources to the circuit.